1. Technical Field
The invention relates to digital mobile telecommunications and, more particularly, to signal receiving circuitry which improves the bit error rate performance in the presence of interference phenomena.
2. Description of the Prior Art
As demand grows for greater capacity in wireless communication systems, the telecommunications industry is looking into different approaches that will make the limited radio frequency (RF) spectrum currently allocated to cellular systems more efficient.
One possible solution to the demand for greater capacity is the digital systems presently being considered to replace or supplement the existing North American cellular system which is an analog system. Also known as advanced mobile phone service or AMPS, the North American cellular system has been standardized in a number of Telecommunications Industry Association (TIA) standards, e.g., TR-45.1, and is based upon analog frequency modulation (FM) technology. This cellular system, however, has the limitation of carrying only one voice signal per cellular radio channel.
Typically, in digital wireless communications systems, bandwidth efficient modulation techniques are used to maximize the amount of information transmitted in the form of digital voice and data channels. These systems do so by reducing the spectral bandwidth required for the transmission of each assigned channel in the radio frequency spectrum.
One digital cellular system is based upon time division multiple access (TDMA) techniques and is defined in TIA interim standard (IS)-54. In this system, typically 3 to 6 users (data channels) share a common 30 KHz channel. Each user transmits data in an assigned time slot that is a part of a larger frame. The gross bit rate of the data to be transmitted over the mobile channel is typically 48.6 kilobits per second (kbps). The modulation method is .pi./4 shifted, differentially encoded, quadrature phase shift keying (DQPSK).
The combination of digital modulation, error-correcting codes, and time-slot interleaving provided by the .pi./4 DQPSK technique reduces the effects of the most common radio propagation impairments. This, in turn, makes the limited RF spectrum currently allocated to cellular systems more efficient, increases subscriber capacity, and triples the voice channel capacity without requiting additional RF-spectrum.
The wireless channel, however, consists of a number of interference phenomena including multipath delay time dispersion or fading, additive white gaussian noise (AWGN), co-channel interference and frequency selective fading. The multipath delay time dispersion in digital communications, for example, causes intersymbol interference and also crosstalk between in-phase and quadrature-phase signals. This interference phenomena causes imperfections on the wireless channel and thus limits the maximum usable transmission rate.
When the multipath delay time dispersion is present at a significant level, the bit error rate performance of a receiver in the system is considerably degraded. The multipath delay time dispersion is characterized by a quality known as delay spread. And delay spread, in turn, is the time interval between the first arriving signal and last significant echo. It is known in the prior an to detect delay spread, as well as co-channel interference. For example, in an article entitled "In-Service Monitoring of Multipath Delay-Spread and C/I for QPSK Signal", published in Proc. IEEE Vehicular Technology Society Conference (VTC) 1992, pages 592-595, S. Yoshida et al. show that the amount of delay spread and also co-channel interference on a channel may be detected by monitoring the value of the in-phase channel and quadrature channel detector outputs. Similarly, in an article entitled A Simple Multipath Delay Time Detection Method for .pi./4 Shift QPSK in Digital Mobile/Portable Communications, published in 41st IEEE Vehicular Technology Conference, May 1991, pages 7-12, B. J. Cho et al. describes an in-service method of measuring multipath delay spread which is applicable to a .pi./4 shift QPSK signal. In Cho's method, a frequency doubling technique is employed that moves all the phase states onto the quadrature channel and monitors the in-phase channel for any distortion. The circuitry or processing required for implementing the method of Cho appears to be rather complex to implement, however. Also, neither Yoshida et al. nor Cho et al. provides compensation for multipath delay spread in their respectively described schemes.
It is therefore desirable for a receiver in a digital wireless system to not only be able to detect distortion due to multipath delay spread, but also be able to compensate for this distortion in order to improve the bit error rate performance of such receiver.